High power fiber lasers and systems have many advantages compared to solid state systems. For example, fiber lasers and systems have more flexibility in transmitting a laser beam, are less susceptible to environmental or external conditions compared to open air and provide more flexibility in directing or focusing the laser beam on an object. Additionally, such systems can be significantly less bulky and mobile compared to solid state systems. However, one limitation of fiber lasers is that current optical fibers are susceptible to damage or degradation as laser power is increased. More robust optical fibers and fiber laser systems are under development. These new optical fibers and fiber laser systems need to be evaluated and tested. Systems and techniques to obtain quantitative data about laser-induced damage in fibers and transmitted beam quality degradation in optical fibers under increasing laser power conditions and various other laser parameters or characteristics are needed for high power fiber laser development.
Techniques for laser-induced damage threshold measurement in transparent solid materials are known. FIG. 1 is a schematic diagram of an example of a prior art system 100 for measuring laser-induced damage thresholds in a solid state material 102. A pulsed laser beam 104 having predetermined parameters is focused by a lens 106 into the solid material 102. The focusing of the laser beam 104 results in a substantial increase in power density of the beam 104 because the power density is equal to the laser power which may be in watts divided by an area which may be in square centimeters. As a result of the focusing, an initial area illustrated by broken line 108 of about a few square centimeters (cm2), for example, may be decreased down to an area illustrated by broken line 110 of about (10−6-10−8) cm2 in this example. Therefore, the initial power density in area 108 which results in damage is increased by focusing by a factor of about 106-108 in the area 110. For example, by focusing a nanosecond laser pulse with energy of about 10 millijoules (10−2 J) and pulse duration of about 10 nanoseconds (10−8 s) into an area of about 10 microns (˜10−6 cm2) a power density of about 10−2 J/(10−8 s*10−6 cm2)=1012 W/cm2. The energy (or the power which is (energy)/(pulse duration)) of laser pulse 104 may then be increased up to the moment the solid material 102 sustains damage. The damage can be observed by different simultaneous events, such as a residual crack in the damaged area; a visible cloud of plasma in the area of damage at the moment of damage; truncation of beam 104 transmitted through the focus area 110; and scattering of visible light of a continuous wave (CW) laser from the damaged area. CW lasers are typically used for damage visualization.
A similar technique to that described above for measuring laser-induced damage thresholds in a solid material cannot be used for measuring laser-induced damage thresholds in optical fibers. Referring to FIG. 2, FIG. 2 is a cross-sectional view of an example of a conventional optical fiber 200 and representation of a laser beamlet 202 propagating through the optical fiber 200. The optical fiber 200 includes a core 204 and a cladding 206 surrounding the core 204. The cladding 206 has a refractive index a little lower than the core. Therefore, the beams 202 in the fiber core 204 experience total internal reflection at the “core-cladding” boundary 208 as illustrated in FIG. 2 and cannot be influenced from outside. Because of the design of the optical fiber 200, the power density distribution of a laser beam in the core 204 will be identical at any cross section of fiber, i.e., it is not possible to focus the beam propagating through the fiber 200 and develop some area with enhanced intensity needed to perform a laser-induced fiber damage test.